The brain is an amazing thing. For our entire lives, it constantly sends electrical signals and messages that form trillions of neural pathways in the brain.
But imagine that those pathways are just like the streets and highways that connect cities and states. Streets along older routes become damaged and may or may not be repaired. Some roadways may wash away or shut down completely.
When you apply this concept to your brain, you can see why memory may begin to fail as we age.